1. Technical Field
The present invention relates to a driving motor controlling device of a construction machine, which has a driving motor connected to a pump and a tank through a swivel joint and a motor control valve for switching a state for connecting the pump and the tank to the driving motor such that the driving motor is controlled to any one of a stop state, a normal rotation state, and a reverse rotation state.
2. Related Art
A driving motor controlling device of a construction machine (hereinafter, referred to as driving motor controlling device) which has a driving motor and a motor control valve for controlling an operation state of the driving motor is known as an example of related art (see Japanese Unexamined Patent Application Publication No. 60-249707). In the driving motor controlling device disclosed in Japanese Unexamined Patent Application Publication No. 60-249707, the motor control valve is integrally formed with control valves for controlling the operation of actuators different from the driving motor, and disposed in an upper body of the construction machine which is higher than a swivel joint. When the construction machine having the driving motor controlling device is operated in an environment which a temperature is low, such as a cold region in winter, since viscosity of hydraulic oil becomes higher upon start-up, a warming up operation is first performed. When the warming up operation is performed, hydraulic oil is circulated from the pump to the tank through the control valves and thus becomes warm.
However, in the construction machine having the driving motor controlling device disclosed in Japanese Unexamined Patent Application Publication No. 60-249707, when the warming up operation is performed, the hydraulic oil is circulated from the pump to the tank through the control valves, but is not circulated from the motor control valve to the driving motor. Accordingly, the hydraulic oil which becomes warm by the warming up operation does not flow between the motor control valve and the driving motor and thus the motor control valve and the driving motor remains cold even when the warming up operation is performed. At this state, when the construction machine begins to be driven, the warm hydraulic oil is rapidly fed to the cold driving motor and thus heat balance of the driving motor is lost. In other words, a portion of the driving motor which is in contact with the warm hydraulic oil rapidly becomes warm, whereas the other thereof remains cold. In the driving motor controlling device disclosed in Japanese Unexamined Patent Application Publication No. 60-249707, when the construction machine begins to be driven after the warming up operation, an operation fault or failure of the driving motor may be caused due to a difference in thermal expansion.
In addition, a counterbalance valve was generally used for preventing cavitation of the driving motor from being created. However, in order to simplify the structure of the valve, the driving motor controlling device disclosed in Japanese Unexamined Patent Application Publication No. 60-249707 realizes a structure for preventing cavitation from being created although the counterbalance valve is not used. In this driving motor controlling device, a main selector valve 14A provided in a circuit for connecting a hydraulic pump 10A to an actuator 12 which is a hydraulic motor is switched by controlling a pilot valve 28. Furthermore, pressure reduction valves 30a and 30b which reduce a set pressure in a conduit line between a pilot valve 28 and a pilot chamber provided at the both sides of the main selector valve 14A according to the reduction in the pump pressure which is caused by the hydraulic oil from the pump 10A are provided.
However, in the driving motor controlling device disclosed in Japanese Unexamined Patent Application Publication No. 60-249707, in order to allow the main selector valve 14A to be switched from a neutral position, the pressure reduction valves 30a and 30b need be formed such that passages 26a and 26b between the remote control valve 28 and the pilot chamber of the main selector valve 14A are sufficiently communicated with each other in an initial state of the switching operation in which a load is not generated in a passage 56 of a discharge side of the pump 10A. In this driving motor controlling device, when the construction machine begins to be self-propelled on a downhill road to reduce the pressure of the passage 56, the pressure may not be sufficiently reduced by the pressure reduction valves 30a and 30b in the initial state of the switching operation and the cavitation may be created in the driving motor.
In order to prevent the cavitation from being created due to the above-described reason, the pressure reduction valves 30a and 30b may be formed such that an opening degree of the passage 56 between the remote control valve 28 and the pilot chamber of the main selector valve 14A is reduced in the initial state of the switching operation. However, when the pressure reduction valves 30a and 30b are formed as described above, a pilot pressure which acts on the pilot chamber of the main selector valve 14A is reduced in the initial state of the switching operation (hydraulic oil fed to the pilot chamber is reduced) and thus the switching operation of the main selector valve 14A is delayed and rising of a driving speed upon the start-up is delayed.